Trailers that are towed by a vehicle in harsh off-road environments are placed under an extreme amount of stress. Trailers that are subjected to high levels of stress must be capable of withstanding the intense motion imparted to the trailer and to the coupling mechanism between the tow vehicle and trailer.
It is the actual suspension system of the trailer that greatly affects the stability of the trailer when towed by a car or truck in a rough, high speed, off-road situation.
Traditional trailer designs use a direct load transfer type of suspension system. With this type of system the load bearing on the trailer is transferred directly to a flexible member, such as a leaf or coil spring. Consequently, with direct load transfer the entire load must be absorbed through the deflection of the flexible member.
The most common suspension designs that utilize direct load transfer are:
(a) beam axles having two semi-elliptic springs fastened to hangers at the front and with shackles or slippers at the rear of the spring; or PA1 (b) trailing arm suspensions, or "neidhart" type suspensions that use rubber cords in compression as the springing medium. PA1 (a) Linear: the load rate is constant throughout the deflection cycle; that is, the change in deflection of the flexible member per unit load change is constant as the flexible member is deflected, or stressed; PA1 (b) Progressive: the load rate increases throughout the deflection cycle; that is, the change in deflection of the flexible member per unit load change decreases as the flexible member is deflected, resulting in an increasing load rate; and PA1 (c) Digressive: the load rate decreases throughout the deflection cycle; that is, the change in deflection of the flexible member per unit load change increases as the flexible member is deflected, resulting in a decreasing load rate. PA1 (a) a stiff suspension for the trailer, whereby the load rate of the trailer exceeds the load rate of the tow vehicle (LR.sub.t &gt;L.sub.v), i.e. a non-compliant suspension arrangement: PA1 (b) a trailer load rate below the load rate of the tow vehicle (LR.sub.t &lt;L.sub.v), i.e. a compliant suspension arrangement:
The problem with these prior art direct load transfer systems relates to the inherent design compromises between load rate and spring size.
The load rate of a suspension system relates to the stiffness and is defined as the change in load divided by the change in deflection. The units of load rate are, for example, kg/cm or lbs/in., where 1 kg/cm =5.6007 lbs/in.
The three most common types of load rates are:
Traditionally, the load rate of the trailer suspension (LR.sub.t) exceeds the load rate of the tow vehicle suspension (LR.sub.v) resulting in high levels of trailer instability when operated in a harsh off-road environment. It is not uncommon for the coupling between the trailer and the tow vehicle to be sheared off in certain situations based on the additional forces exerted on the coupling due to the extreme relative motion between the tow vehicle and the trailer imparted by bumps etc.
Currently, the only way to overcome this instability problem is by making the trailer suspension more compliant than the tow vehicle suspension. A trailer is said to be more compliant than the tow vehicle when the trailer load rate is lower than the tow vehicle load rate. If such a design is effected, the trailer having the lower load rate, will be able to overcome, to a certain extent, the added loads transmitted through the coupling mechanism.
However, if the load rate of the trailer suspension is lowered to make it more compliant a very large spring would be necessary to accommodate the additional deflection requirements based on a given load. In particular, the spring must be extremely long so as not to be over stressed.
In summary, there are currently two load rate configurations available with direct load transfer suspension systems: